Capstan winches of the single drum and double drum types have long existed, but they suffer from the problem of causing the very rapid wear of the cable being manipulated. This is due to their very principle of operation. The cable is driven by adherence. As the tension that is applied to the cable increases, the cable stretches and its linear speed increases accordingly. Since the driving grooves with which the winch drums are equipped are totally rigid, the result is relative slippage between the cables and the grooves.
In the attempt to relieve these problems many modifications have been proposed in the prior art. Improvements have been contributed to this type of winch in order to limit slippage, but they have not entirely eliminating it.
Many types of winches exist. Particularly the drum winches referred to above, of the capstan type or of the type called “traction winches” according to Anglo-Saxon terminology, and the multi-pulley type.
In French Patent No. 90 14 612, published under the number FR 2 669 701 B1, entitled, “Poulie d'adhérence,” the applicant has proposed a pulley permitting a traction force to be applied to a cable. The pulley comprises a wheel and an endless belt wound on the peripheral surface of the wheel to support the cable to be driven. The endless belt is made up of a succession of shoes spaced apart which can slide with friction on the wheel, each shoe being connected to the next by an elastically stretchable connector whose longitudinal modulus of elasticity and/or dimensions are chosen such that, in service, the elongation of the endless belt corresponds to that of the cable under a predetermined maximum tension.
For a more detailed description of the characteristics of this device it will be beneficial to consult the above-mentioned patent.
These arrangements make it possible to eliminate virtually all slippage. Shoes embodying the technology taught by the patent have since then been made and used. They completely respond to the purposes which were the aim of the above patent, at least for a well-defined range of applications.
In fact, the technology employed does not permit the manufacture of winches capable of lifting loads exceeding 50 tonnes (about 490 kN). Moreover, this type of winch is very sensitive and somewhat unstable in use. Lastly, great heat can develop in certain circumstances due to the slippage of the endless belt, especially on the edges of the drum.
Nowadays the oil exploration industries, in particular, need winches capable of dragging very heavy loads, typically equal to or greater than 200 T (about 1960 kN), and this in very deep waters (2000 m to 3000 m, even 10000 m for certain applications). Only synthetic cables (whose apparent weight in water is very low, even zero) can be used for this type of application. However, these cables are very expensive, which excludes the use of classical winches. Indeed, as stated above, winches of this type would wear them out too fast.
Lastly, some cables have a complex structure. In fact, in addition to their classical applications, cables are sometimes employed in related applications: energy transport, for example electrical energy (electric carrier cables), or the transmission of analog signals or digital data, for example internal optical fibers (cables known as optical carriers). It can be understood that this type of cable is particularly expensive and that premature wear is economically unacceptable.